1. Field of the Invention
The present invention relates to an apparatus and process for producing a vinyl chloride homopolymer or vinyl chloride-based copolymer (hereinafter generically referred to as "vinyl chloride-based polymer"), and more particularly to an apparatus and process for producing a vinyl chloride-based polymer wherein polymerization of vinyl chloride monomer or a vinyl chloride-based monomeric mixture (hereinafter generically referred to as "vinyl chloride-based monomeric material") dispersed in an aqueous suspension for polymerization reaction is carried out while circulating the aqueous suspension through a heat exchanger which is arranged separately from a polymerization vessel.
2. Description of the Prior Art
Heretofore, suspension polymerization of a vinyl chloride-based monomeric material has been carried out by a process in which water, the vinyl chloride-based monomeric material, a polymerization initiator and a dispersant, together with other various additives as required, are placed into a polymerization vessel equipped with a jacket and a reflux condenser, and cooling water is passed through the jacket and reflux condenser to remove the heat of polymerization, thereby controlling the temperature of the reaction system to a predetermined temperature.
With the trend toward use of larger polymerization vessels in recent years, however, the heat transfer surface area of the polymerization vessel per unit quantity of polymerization reaction mixture has been decreased. Therefore, it has become difficult to achieve effective removal of heat by conventional cooling means. To cope with this problem, various methods have been hitherto adopted, for example, a method by using a larger reflux condenser so as to increase the heat-removing capability, or a method in which cooling water to be passed through the jacket for the polymerization vessel is passed through a refrigerator so as to achieve more powerful cooling.
However, the former method has the drawback that carry-over arises from foaming of the polymerization reaction mixture, leading to polymer scale deposition inside the apparatus, or the number of fish-eyes in the resulting polymer product is increased. The method also involves another drawback that the heat-removing capability is liable to be lowered, and a further drawback that the method cannot be used from the start of polymerization because application of the method at a stage of low polymerization degree leads to the formation of coarser polymer particles.
The latter method, on the other hand, is too expensive and therefore infeasible economically. Furthermore, in the case of producing a vinyl chloride-based polymer with high polymerization degree, the difference between the polymerization temperature and the temperature of cooling water should not be large; in such a case, therefore, the latter method is of no use for shortening the polymerization time.
As an alternative to the above two methods, a method has been proposed in which an aqueous suspension mixture is circulated through a cooler arranged outside of a polymerization vessel (See Japanese Pre-examination Patent Publication (KOKAI) Nos. 54-24991, 56-47410, and 58-32606, and Japanese Patent Publication (KOKOKU) No. 64-11642). This method is extremely efficient in that the heat transfer surface area of the cooler coming into contact with the aqueous suspension mixture can be enlarged.
In a polymerization apparatus using the external-circulation cooling system as described above, the aqueous suspension mixture (polymerization reaction mixture) circulated is returned into the polymerization vessel through a nozzle arranged in a gas-phase zone of the polymerization vessel. The returning of the aqueous suspension mixture is accompanied by spattering of the mixture, adhesion of polymer slurry or the like. Therefore, this approach has the drawback of increased deposition of polymer scale on the inside wall of the polymerization vessel, in the area ranging from the gas-phase zone to the gas-liquid interface portion. The increased scale deposition imposes a serious problem on practical polymerizing operation.